Magnetostrictive vibrator



G. W. PIERCE IAGNIEFI'OS'IRICJ'I'IVE VIBRATOR Original Filed Aug. 17.'1928 Oct. 11, 1932.

17: vs iaoi' 6602196 ila'ei'o'e Patented Oct. 11, 1932 UNITED srAr-ssPATENT OEFICE.

GEORGE WASHINGTON PTERCE, OF CAMBRIDGE, MASSACHUSETTS MAGNETOSTRIGTIVEVIBRATOB Original application filed August 17, 1828, Serial No. 800,849.Divided and this application filed May 9,

' 1930,'- Serial No. 451,051.

The present invention'relates to vibrators, and more particularly tomagnetostrictive vibrators.

A magnetostrictive vibrator comprises a magnetostrictive core disposedin an'electromagnetic field, such as may be established by passing anelectric current through a field coil or winding. The core may be in theform of a rod or tube, or any other desired 1 form. Any material havingsuitable properties may be used for the core, but the mate.- rial shouldobviously be characterized by comparatively large magnetostriotiveeffects and comparatively .low vibrational decrement.

When stimulated magnetically by the field, the core .becomes veryslightly mechanically deformed 'or distorted by magnetostriction.

The resulting increment of deformation may be alengt-hening, or ashortening, 'or some other distortion, depending on the material and onthe polarity of the increment of the magnetic field. This action of themagnetic field upon the core will, for brevity, be here,- inafter termedstimulation. Conversely, when the vibrator is mechanically deformed ordistorted, it will react magnetically upon the magnetic field bymagnetostriction, with an increment of magnetization depending upon thenature of the preexisting magnetic field and the mechanical deformation,and this -will produce its effect upon the electric current or voltagein the coil. This reaction will, for brevity, be hereinafterreferred to3 5 as the response. The mechanical deformation is produced by excitinreversible internal stressesin the core, and the core readily recoversupon the withdrawal of the deforming forces. v

If the current or voltage is alternating, the

electromagnetic field created thereby will also be alternating. The corewill, therefore, in-

crease and decrease in length, let us say, many times a second, everyvariation-in the current producing its stimulative effect on the core,and every deformation of the core producing its reaction response uponthe current. The core will, in consequence, freely vibrate mechanicallyby ma gnetostriction. Ordinarily,

these vibrations will be quite small. When the alternating frequency isvaried so as to assume a value close to, or substantially the same as,the natural frequency of mechanical vibration of the core, however, theamplitude of vibration of the core, though still small, becomesrelatively quite large. The core will then react inductively on the loadto render its consumption of power critical as to frequency forfrequencies near the free frequency of the core. There will usually bemore than one specific frequency of magnetization at which the core willthus resonate; for, in addition to one or more natural fundamentalfrequencies of mechanical vibration, it has also frequencies ofvibration de termined by the operation of the core in halves, thirds,fourths, fifths, etc.

The chief object of the present invention is to provide new and improvedmagnetostrictive vibrators.

Other objects will be explained hereinafter and will'be particularlypointed out in the appended claims, it being understood that it isintended to set forth, by suitable expression in the claims, all thenovelty that the invention may possess.

The invention will now be explained in connection with the accompanyingdrawing, in which Fig. l is a perspective of a vibrator constructed inaccordance with a preferred embodiment of the present invention; Fig. 2is a section and Fig. 3 is an elevation of modifications; and Flg. 4 isa plan of the apparatus shown in Fig. 3.

To illustrate the principle of the invention, the magnetostr-ictive coreis shown in Fig. 2 axially positioned within, and driven by, an

inductive and resistive solenoid field coil 10,

with clearance to permit free vibrations. The coil 10 is providedwithconductors 12 and 14 by which it may be connected, for simplicity, inseries with a source of alter nating electromotive force, such asan-alternating-current generator 16. A local battery 18 (shown in Fig. 2in series with the source 16 and the windin 10) ap lies a steadymagnetizing or po arizing field to the core, over which the alternatingfield produced by the generator 16 is superposed. The alternating fieldis preferably smaller than the steady field, in order that thecombinedfields may not, at any time, fall to zero. The batteriemay be dispensed.with,v andthe core may magnetiz electromagnetically by a local sourcesending a polarizing current through a separate polarizing .coil, oritmaybepermanently magnetized, instead, if'the vibrator has sufiicientma'g'netic retentiveness,'- or the polarization may be produced by a.polarizing magnet supported near the vibrator, or the batteryand apermanently magnetized core vmay -be employed together. These remarks,"it will be understood, are applicable to all the various typesofcomposite vibrators illustrated and described herein.

. In-order not to complicate the showing of Fig. 2,.no tuning condenseror other means is illustrated therein for tuning the circuit or 'varyingthefrequency of the alternating current; flowing therein, particularlyas the core may itself be a tuned elementfof very low decrement, therebydispensing 'with or supplementing-electrical tuning of the circuits. Animportant feature of the invention, as described in applicationSerialNm- 300,249, filed August 17, 1928, of which the presentapplication is a division, contem plates-the use of a tuned system, asgreat frequency selectivity is, thereby attained.

The frequency of a particular mode-of vibration of a rod or bar isdetermined by its elasticity, length. and density. For some modes :ofvibration, the frequency is aifect ed also by the width, thickness,radius, and the like, of the rod or bar; Different bodies,

therefore, havefdiiferent magnetostriictive' properties. Alloyscontaining nickel; chromium, cob It and steel, in proper proportions,have comparatively large magnetostriction, Thus, if the vibrator is inthe .formlof a rod or tube, ofsmall diameter, the period of vibration isnearlyproportional to the length of the rod or tube. A rod ofnickelsteel, for exam le,. known in the trade as stoic metal, a'ving adiameter of one-x half centimeter and a length of ten centimeters, has afundamental period of longi-T tudinal vibration of about 1/21,000 of asecond. A rodof the same diameter ten times as long (100 centimeters)has aperiod about the fundamental 1/2,100 of a second. Rods of the samedimeter and the same two respective lengths, but constituted of an'alloy' of iron and chromium in a particular proportion, have eriods cf1/27,000 and .1/2',700 of a'secon respectively. These results areconsistent with the fact that the two materials-have differentelasticities and densities As ordinar metals have their elasticity anddensity s ightly modifiable by changes intemperature, furthermore, suchtemperature c anges introduce small variations in the natural periodofmechanical vibration wide limits.

of such bodies. As is explained in the said application, substantiallyconstant frequency, independent of the temperature, may be obtained bymaking the vibrator of material .having a coefficient of the ratio ofelasticity to density that varies as little as possible with variationof temperatures Certain alloys of steel, nickel and chromium are knownto possess substantially constant coefiicients 'of "frequency withvariations of temperature. One such alloy, constituted of 52% iron, 36%nickel. and 12% chromium, is-practically independent .of temperature. Ihave found that a rod of nickel, chromium and steel has a period thatis-also practically in-' dependent of magnetic field strength over Anickel-iron alloy of about nickel, and another alloy of about nickelhavepracticallya zero temperature coefiicient of frequency of vibration.

The elasticity and the density determine also the velocity ofpropagation, of sound waves in the material. In general, the

higher the velocity of the sound, the higher the frequency. As is wellknown, various metals and alloys have various velocities of sound.

. Various alloys, furthermore, have different temperature coefficientsof frequency of vibration. The values of some ofthese velocitiesandcoefficients, which I have determined experimentally, are presentedin my recently published paper on Magnetostriction oscillators,appearing in the Proceedings of the American Academy of Arts andSciences, vol..-

63, No. 1, April, 1928.

I have discovered a number-of alloys that have positive coefficients(for which the fre- A nickel-iron alloy of about 35% to 40% nickelhasthe largest positive coefficient;

I have found that either a reduced or an enhanced temperaturecoeflicient of frequency lit" vibration can be obtained By theemployment I of a composite vibrator made up of two or more materialsunitedmechanically, as by -welding or soldering; A substantially zerotemperature 'coeflicient is desired for many purposes. Compositevibrators .of this n'a- I 1 ture may, indeed, be produced having ire--uency characteristics very difierentfrom. t at of one of the constituentmaterials alone;

For example, a material of very low sound velocity, such aslead'or typemetal, may be united with some magnetostrictive alloy to obtain a lowfrequency of longitudinal vibration with, agiven lengthof vibrator.

As another example, a material of positive frequenc to give a vibratorof zero temperature' coe dent of frequency. Again, a material whichexpands on increase of magnetization may be, united with a materialwhich contracts or .which is neutral with'increase of magnetization toproduce a vibrator that operates transversely, so that when magne- 1tlzed lengthwise the composite vibrator executes flexural vibrations.Such vibrations are of very low frequency compared with theIfJreJquency-Of longitudinal vibrations of the o y. k

The illustrated vibrator is composite and is so designedas to yieldtransverse vibrations upon at low frequency, and serves as a veryeffective frequencymeter. The composite bar is constituted of twodissimilar materials 11 face-to-face contact, along their adjoiningsurfaces 15 into a rectangular-bar stri as shown, in any desired'way, asby wel ing,

soldering, or pressing. At least one of these materials ismagnetostrictive. The other may or may not be magnetostrictive,depending t e use to Wl'llCll it is desired to put the vibrator. Thus,one of the materials may be constituted oflead, type metal or variousalloys, if low frequency is desired. If one of the materials is of stemmetal or other metal havin a positive temperature coeflicient, and theot er of nickel or other metal having a negative temperaturecoefficient, the vibrator will have a frequency of vibration that ispractically independent of temperature.

The vibrator is sup orted rigidly at one end upon a base 19 an issurrounded by the driving coil 10. When, an alternating current is madeto traverse the coil 10, the top-endof the bar oscillates or vibratestransversely between the limits 29 and 31, due to the unequalmagnetostrictive operation of the-two members 11 and 13 of the bar.

It is possible to employ a plurality of such a composite bars or reeds,all secured to a common base 19, and having a single driving coilsurrounding the group of reeds, as shown in Figs. .3 and 4'. This devicemay serve as a requency meter in that, when analternatin currenttraverses the coil, one particular reed, whichis resonant to thefrequency of the current, may be set into vibration while the othersremain at rest. A calibration of these reeds as .to frequency thusserves to determine the frequency of the current.

It will be understood that, in all cases, at

least one of the materials of the'composite vibrator ismagnetostrictive, and the other material has a compensating propertywhich causes a shifting of the-characteristics of the vibrationstoward-the desired result.

pointed out above, the

Thus, as has been compensation may'consist in a modified hevibratorconsisting of a nickel tube alone that' would give 1000 cycles persecond would have to be 2 meters 1on which is inconvenient- 1y large.All that 1s necessary, in order to "obtain a vibration frequency of 1000cycles per second with a convenient, one-meter tube, is to fill it.

As anotherillustration, a single bar of invar, mounted as shown in Fig.2, when excited by the surrounding coil 10, would execute vibrations inthe direction of the length of the bar, and if this bar were ten centimeters long, the frequency of vibration would be about 12,000 cycles persecond. If, now, the single invar strip be replaced by a composite stripconsisting of invar on one s1de and Monel metal, or brass, or almost anyother material, on the otherside, the lengthwise magnetization will makeone of the component elements vibrate more or less than the other, or inan opposite sense to the other, and will give to the system a frequencyof transverse vibration which ma be relatively very small, for example,cyc es per second. I have built a series of composite reedvibrators ofthis character with periods ranging from 50 cycles per second to about5,000 cycles per. second, thus supplementing and extendmg the frequencyspectrum of magnetostrictive vibrators.

A. third compensatory action, which is highly useful infrequency-measuring or he quency-controlling devices, is the eliminationof the effect of temperature on the frequency of vibration ofmagnetostrictive vibrators. This is attained, according to the presentinvention, as before explained, by combining a member having a positivetemperature coefficient of frequency with a compensating member having anegative coeflicient. The physical union of the two materials appears tohave little, if any, effect upon their magnetostrictive properties. Forthe positive mem ber, I prefer to usea nickel-steel alloy, such as hasbeen referred to above, particularly a nickel-steel alloy of about thecomposition of invar. This is-because of its availability anddurability,

and because it may be readily welded to the other material. Invar hasthe "but I may also effectively use Monel metal or nickel, or almost anyother material of su1table elastic qualities, provided it has a negativetemperature coefiicient of frequency.

The relative thickness of the two bodies may be readily so chosen as toeffect almost comiplete compensation for the efiects of temper- 'atureon frequency of the composite system.

Ihave derived theoretically the following formula for the frequency f ofvibration of the longitudinally composite vibrator f r (1) where w and'w are the respective weights, of the two bodies, and f. and f are therespective frequencies of the two bodies, it bea ing understood thatthetwo. bodies are of the same length. This theoretical formula changeof. temperature, then the condition for a zero temperature posite systemis that alibi-fl allwllfll so that, when the frequenciesand tempera-'-ture coeficients of frequency ofthe two macoefiicie'nt of the comterialsare given, Equation (2) permits a ready calculation of.the weights ofthe two- 'materials which, associated together,- will give a zerotemperature coeflicientof frequency. i As anexample, I find that aninvarstrip increases in frequency 2.2 cycles in 10,000

per degree centigrade change in temperature 1 while a. composite bar ofinvar with an equal longitudinal str'i thickness of brass welded to ithas this coe cient reduced to 0.44 cycles 2) which, indicate to thoseskilled in the art, '1

t e proper preliminary desi ofone'form of the novel vibrator,suc .designtobe further modified by experimental tests and subsequent adjustmentif'necessary.

To persons skilled in the art many other applications andmodificationsof the apparatuswill occur, and no eflior't has ,here beenmadetobe exhaustive. l v

What is claimed is 2' .1. Apparatus of the kind described comprising abase, and a plurality of magnetostrictive vibrators of differentvibration frequenc attached to the base.

2. pparatus of the kind magnetostrictive vibrators of dififerent-vibration frequency attached to the base, each vibrator, being.constituted of mechanically I connected members of different.'magnetostrictive material.

4 3. A composite magnetostrictive vibrator constituted of mechanicallyconnected members of different'magnetostrictive material,

and a base to which the vibrator is ,zittached.v

4. The combination of means for producing a magneticfield, and twomembers rig-j idly mounted at one end and respectively operable tocontract and elongate when subjected to-said field.

--In{ testimony whereof, I have hereunto subscribed my name;

in 10,000 per degree centigrade. The brass has a negative coelficientwhich particularly.

compensates for the positive coefficient of invar. The particularthicknessemployed in this experiment .was' not suflicien-t to completelycompensate, but b using a piece of brass'about 1.3 times .the t icknessof .the 'invar, the compensation may be made com plete.

separate materials, do' not give an exactly zero temperature coefiicientoffrequency, it is aneasy'matter, by trial and by-filing away one. ofthe materials, to adjustthe system to any required accuracy.

In practice, where the calculations, on account of insuflicientknowledge of the It is readily understood from the abovevdisclos'uresthat a large variety of materials,'- y :when given properrelative dimensions, may

be usedto replace the brass,' andto some exf tent also there is libertyof choice among a limitedlrange of known materials with posie tivetemperature coeflicients to replace the in- 1. a

var-and form a combination of negligible temperature coefficients offrequency. Because of this liberty of choice of materials it is'preferred to describe the methods of temperature compensation in termsof the results obtained and the guiding formulae '(1) and describedc0111. prising a base,-and a plurality of composite" GEORGE W. PIERCE. f

